Opa3, a novel regulator of mitochondrial function, controls thermogenesis and abdominal fat mass in a mouse model for Costeff syndrome.

The interrelationship between brown adipose tissue (BAT) and white adipose tissue (WAT) is emerging as an important factor in obesity, but the effect of impairing non-shivering thermogenesis in BAT on lipid storage in WAT remains unclear. To address this, we have characterized the metabolic phenotype of a mouse model for Costeff syndrome, in which a point mutation in the mitochondrial membrane protein Opa3 impairs mitochondrial activity. Opa3(L122P) mice displayed an 80% reduction in insulin-like growth factor 1, postnatal growth retardation and hepatic steatosis. A 90% reduction in uncoupling protein 1 (UCP1) expression in interscapular BAT was accompanied by a marked reduction in surface body temperature, with a 2.5-fold elevation in interscapular BAT mass and lipid storage. The sequestration of circulating lipid into BAT resulted in profound reductions in epididymal and retroperitoneal WAT mass, without affecting subcutaneous WAT. The histological appearance and intense mitochondrial staining in intra-abdominal WAT suggest significant 'browning', but with UCP1 expression in WAT of Opa3(L122P) mice only 62% of that in wild-type littermates, any precursor differentiation does not appear to result in thermogenically active beige adipocytes. Thus, we have identified Opa3 as a novel regulator of lipid metabolism, coupling lipid uptake with lipid processing in liver and with thermogenesis in BAT. These findings indicate that skeletal and metabolic impairment in Costeff syndrome may be more significant than previously thought and that uncoupling lipid uptake from lipid metabolism in BAT may represent a novel approach to controlling WAT mass in obesity.

Specific deficits in visual electrophysiology in a mouse model of dominant optic atrophy.

Autosomal dominant optic atrophy (ADOA) is a slowly progressive optic neuropathy caused by mutations in the OPA1 gene. OPA1 is ubiquitously expressed and plays a key role in mitochondrial fusion. Heterozygous Opa1 mutant mice (B6; C3-Opa1(Q285STOP)), have previously been reported to develop visual defects and optic nerve changes. In this study, in vivo visual electrophysiological testing (ERGs and VEPs) was performed on 11-13 month old B6; C3-Opa1(Q285STOP) mice (n = 5) and age/sex matched wildtype littermate controls. Full intensity series were recorded in response to brief (4 ms) single flash stimuli delivered in a Ganzfeld dome under dark- and light-adapted conditions. The major ERG components (a-wave and b-wave) showed no detectable difference from wildtype in the amplitude or implicit time of dark-adapted ERGs across the full intensity range tested. This was also true for the components of the dark-adapted VEP. However, the light-adapted ERG responses revealed a significant reduction in the photopic negative response (PhNR) amplitude in Opa1(+/-) animals relative to wildtypes at the brighter intensities tested. Elements of the light-adapted VEP were also abnormal in mutant mice. Overall Opa1(+/-) mice display functional deficits in electrophysiology that are consistent with ganglion cell dysfunction. These deficits may correlate with a reduction in the dendritic arborisation of retinal ganglion cells, which has been previously reported to occur at a similar age in the same mutant mouse line (Williams et al., 2010). The functional phenotype we have described in this mouse model may be useful in the robust and accurate assessment of potential treatments for ADOA.

A missense mutation in the murine Opa3 gene models human Costeff syndrome.

Opa3 mRNA is expressed in all tissues examined to date, but currently the function of the OPA3 protein is unknown. Intriguingly, various mutations in the OPA3 gene lead to two similar diseases in humans: autosomal dominant inherited optic atrophy and cataract (ADOAC) and a metabolic condition; type 3-methylglutaconic aciduria (MGA). Early onset bilateral optic atrophy is a common characteristic of both disorders; retinal ganglion cells are lost and visual acuity is impaired from an early age. In order to investigate the function of the OPA3 protein, we have generated a novel ENU-induced mutant mouse carrying a missense mutation in the OPA3 gene. The heterozygous mutation in exon 2, causes an amino acid change p.L122P (c.365T>C), which is predicted to alter tertiary protein structure. In the heterozygous state, the mice appear uncompromised however; in the homozygous state mice display some of the features of MGA. Visual function is severely reduced, consistent with significant loss of retinal ganglion cells and degeneration of axons in the optic nerve. In the homozygous optic nerve, there was evidence of increased mitochondrial activity, as demonstrated by the increased presence of mitochondrial marker Cytochrome C Oxidase (COX) histochemistry. Mice homozygous for the opa3(L122P) mutation also display a severe multi-systemic disease characterized by reduced lifespan (majority dying before 4 months), decreased weight, dilated cardiomyopathy, extrapyramidal dysfunction and gross neuro-muscular defects. All of these defects are synonymous with the phenotypic characteristics of Type III MGA found in humans. This model will be of major importance for future studies of the specific function of the OPA3 gene.

Impaired processing of local geometric features during navigation in a water maze following hippocampal lesions in rats.

Hippocampal damage impairs navigation with respect to information provided by the shape of an arena. Recent evidence has suggested that normal rats use local geometric information, as opposed to a global geometric representation, to navigate to a correct corner. One implication of this pattern of results is that hippocampal lesions may impair processing of 1 or more of the local geometric features of an environment. The authors therefore investigated the effects of hippocampal cell loss in rats on navigation to a hidden goal with respect to a variety of local cues in an environment with a distinctive shape. Rats with lesions of the hippocampus were impaired in discriminating a right-angled corner from its mirror image. However, they were able to use cues provided by an acute-angled corner (Experiment 1) or a local polarizing cue (Experiment 2). In contrast, lesioned rats were impaired in discriminating long versus short walls (Experiment 3). Results indicate that the hippocampus plays a role in disambiguating locations by processing (metric) information related to the distance between corners.

Secondary mtDNA defects do not cause optic nerve dysfunction in a mouse model of dominant optic atrophy.

PURPOSE: The majority of patients with autosomal dominant optic atrophy (DOA) harbor pathogenic OPA1 mutations and certain missense mutations, mostly within the GTPase domain, have recently been shown to cause multiple mitochondrial DNA (mtDNA) deletions in skeletal muscle. This raises the possibility that the optic neuropathy could be the result of secondary mtDNA defects accumulating within retinal ganglion cells (RGCs). To explore this hypothesis, the authors looked for evidence of mitochondrial dysfunction in a mouse model of DOA and documented the visual and neurologic progression in aging mutant mice. METHODS: Visual function was assessed with a rotating optokinetic (OKN) drum at ages 13 and 18 months and neurologic phenotyping was performed using the primary SHIRPA screen at age 13 months, comparing mutant Opa1(+/)(-) mice with wild-type C57Bl/6 mice. The presence of cytochrome c oxidase (COX) deficiency and multiple mtDNA deletions was investigated in gastrocnemius muscle and eye specimens harvested from 2- and 11-month-old Opa1(+/+) and Opa1(+/)(-) mice. RESULTS: At age 13 months, Opa1(+/)(-) mice had a statistically significant reduction in OKN responses compared to C57Bl/6 controls with both 2 degrees and 8 degrees gratings (P < 0.001). At age 18 months, the difference between the two groups was significant for the 8 degrees grating (P = 0.003) but not for the 2 degrees grating (P = 0.082). Opa1(+/)(-) mice did not exhibit any significant neuromuscular deficits and no COX deficient areas or secondary mtDNA deletions were identified in skeletal muscle or the RGC layer. There was also no evidence of significant mtDNA depletion or proliferation in skeletal muscle from Opa1(+/)(-) mice. CONCLUSIONS: COX deficiency and mtDNA abnormalities do not contribute to optic nerve dysfunction in pure DOA.

OPA1 deficiency associated with increased autophagy in retinal ganglion cells in a murine model of dominant optic atrophy.

PURPOSE: To examine retinal ganglion cell (RGC) and axonal abnormalities in an ENU-induced mutant mouse carrying a protein-truncating nonsense mutation in OPA1. Mutations in the OPA1 gene cause autosomal dominant optic atrophy (ADOA) in which loss of RGCs followed by myelin degeneration in the optic nerve leads to progressive decrease in visual acuity. METHODS: Ultrastructure of the optic nerve was examined in heterozygous mutants and wild-type littermate controls at 6, 9, and 24 months using electron microscopy. The RGC layer was examined at 6 and 24 months. RESULTS: There was an increase in the number of autophagosomes in the RGC layer in heterozygous mutants compared with wild type at 24 months. Signs of optic nerve degeneration were seen as early as 9 months in Opa1(+/-) mice, with more severe degeneration by 24 months. By 24 months, degeneration of axons was also seen in control mice. Numbers of opaque mitochondria in the Opa1(+/-) mice increased at 6 and 24 months, possibly representing an increase in the density of cristae to fulfill the energy requirements of the axon. In addition, mitochondria with vesiculation of the inner membranes, similar to the mutant mitochondria described in a mouse model of Charcot-Marie-Tooth type 2A, were observed. CONCLUSIONS: Mutations in OPA1 cause pathologic changes to optic nerve axons that are similar to, but occur earlier than, age-related degeneration. Increased autophagy is likely to result from an increase in abnormal mitochondria and could be one mechanism contributing to RGC loss and subsequent optic atrophy seen in ADOA.

Opa1 deficiency in a mouse model of autosomal dominant optic atrophy impairs mitochondrial morphology, optic nerve structure and visual function.

OPA1 is a ubiquitously expressed, nuclear dynamin-related GTPase, targeted to the inner mitochondrial membrane, which plays a role in mitochondrial fusion. Mutations in the OPA1 gene on chromosome 3q28-qter are associated with autosomal dominant optic atrophy (ADOA), the most common inherited optic neuropathy, in which retinal ganglion cells (RGCs) are lost and visual acuity is impaired from an early age. We have generated a novel ENU-induced mutant mouse carrying a protein-truncating nonsense mutation in opa1 in order to explore the pathophysiology of ADOA. The heterozygous mutation, B6; C3-Opa1(Q285STOP), located in exon 8 immediately before the central dynamin-GTPase, leads to approximately 50% reduction in opa1 protein in retina and all tissues on western analysis. The homozygous mutation is embryonic lethal by 13.5 days post coitum, demonstrating the importance of Opa1 during early development. Fibroblasts taken from adult heterozygous mutant mice show an apparent alteration in morphology, with an increase in mitochondrial fission and fragmentation. Heterozygous mutants show a slow onset of degeneration in the optic nerve electron microscopy. Furthermore, they demonstrate a functional reduction in visual function on testing with the optokinetic drum and the circadian running wheel. These findings indicate that the opa1 GTPase contains crucial information required for the survival of RGCs and that Opa1 is essential for early embryonic survival. The Opa1 +/- mice described here provide a means to directly investigate the cellular pathophysiology of OPA1 ADOA.